Macular pigment is the yellow pigment situated in the central portion of the human retina. The absorption spectrum for the pigment has a peak for light of a wavelength of 460 nm, and zero for light of a wavelength of 540 nm, so that the pigment absorbs significant amounts of the shorter wavelength light, whilst having little or no effect on light of the longer wavelength.
The highest concentrations of macular pigments are to be found in the region of the retina which has a very high number density of cone receptors, and is coupled to a disproportionately large area of the visual cortex, giving that region a high degree of visual acuity.
It has been proposed that the macular pigment protects the retina against harmful effects of short wavelength radiation, and accordingly much effort has been devoted to measuring the optical density of macular pigment in various subjects in order to determine whether there is any correlation between irregularities in the amount of macular pigment present in various subjects and certain sight defects.
To that end, a flicker photometer projects green and blue light (respectively of wavelengths of 540 nm and 460 nm) into a subject's eye in an alternating sequence. The subject is able to vary the intensity of light of one of those wavelengths until, if the frequency of alternation is sufficiently high, the subject perceives a minimum or no flickering of light projected into his or her eye. When this condition is achieved, the relative intensities of light of the different colours are calculated and that calculation is used to provide an indication of the optical density of macular pigment.
One example of such a photometer is described in the article "Comparison of Macular Pigment Densities in Human Eyes" (R A Bone and J M B Sparrock: Vision Res. Vol. 11, pp 1057-1064. Pergamon Press 1971).
In that photometer, a subject could vary the intensity of light from one or two beams by altering the position of a wedge-shaped filter. The position of the filter relative to the beam is related to the depth of filter through which the beam travels, and hence the degree of attenuation of the beam.
However, movement of the filter is achieved using a mechanical linkage. However, such a linkage can be cumbersome, and does not facilitate fine adjustments of the wedge position over the necessary range of wedge movements.